Devices for conducting electrical signals between two members that are rotatable relative to one another are well known in the art. Such devices, generically known as rotary joints, include slip-rings and twist capsules, inter alia. Slip-rings are typically used when unlimited rotation between the members is desired, while twist capsules are typically used when only limited rotation between the members is required.
Conventional slip-rings typically employ sliding electrical contacts between the members. These work well in most applications, but have inherent weaknesses that constrain electrical performance at higher frequencies. The physical construction of electrical contacts typically presents impedance-matching and bandwidth constraints that degrade signal integrity. In addition, sliding electrical contacts inherently generate wear debris and micro-intermittencies that complicate the recovery of data from digital signals and that negatively impact signal integrity and service life. These issues are exacerbated by fast edge-rise and fast edge-fall times of high-speed digital signals, which constrain the high-frequency performance of slip-rings.
Various techniques exist that extend the use of contact-type slip-ring technologies to higher frequencies and higher data transmission rates. These techniques are representatively shown and described in the following patents:
Pat. No.TitleU.S. Pat. No. 6,956,445 B2Broadband High-FrequencySlip Ring SystemU.S. Pat. No. 7,142,071 B2Broadband High-FrequencySlip Ring SystemU.S. Pat. No. 7,559,767 B2High-Frequency Drum-StyleSlip-Ring ModulesU.S. Pat. No. 6,437,656 B1Broadband High Data Rate AnalogAnd Digital Communication Link
Contact-type slip-ring technologies exist that allow high-speed transmission of digital electrical signals at data transmission rates on the order of 10-gigabits per second (“Gbps”). However, the problems inherent in sliding electrical contacts (e.g., wear debris generation and contact lubrication issues) present long-term constraints to reliability.
The present invention enables the transmission of high-frequency electrical signals between a rotor and stator without sliding electrical contacts. The following patents disclose aspects of existing non-contacting rotary joint systems:
Pat. No.TitleU.S. Pat. No. 5,140,696 ACommunication System For TransmittingData Between A Transmitting AntennaUtilizing Strip-Line Transmission LineAnd A Receive Antenna In RelativeMovement To One AnotherU.S. Pat. No. 6,351,626 B1System For Non-contacting Of ElectricalEnergy Or Electrical SignalsU.S. Pat. No. 6,433,631 B2RF Slipring Receiver For A ComputerizedTomography SystemU.S. Pat. No. 6,798,309 B2Arrangement For Transmitting ElectricalSignals And/Or Energy Between Parts ThatCan Be Rotated In Relation To Each OtherU.S. Pat. No. 6,614,848 B2Device For Transmitting SignalsBetween Moving PartsU.S. Pat. No. 7,466,791 B2Data Transmission System For ComputerTomographsU.S. Pat. No. 7,880,569 B2Rotating Data Transmission Device
Such non-contacting systems include devices to recover electromagnetic energy transmitted across space between a signal source and a signal receiver. In radio frequency (“RF”) communications systems, such devices are called antennas (or antennae), and typically operate in the classical far-field electromagnetic radiation of free space. In contrast, the present invention provides rotary joints that utilize the electromagnetic near-field to effect electrical communications across very short distances. Devices that recover energy from the electromagnetic near-field are termed “field probes”, or simply “probes”.
Devices intended to function in the reactive near-field of an electromagnetic source take different forms than their far-field counterparts, with magnetic loops, voltage probes, and resistively-loaded dipoles being known in the art. Near-field applications include RF ID tags and secure low-speed data transfer, which utilize magnetic induction in the near-field. As used herein, a “probe” is a structure that operates in the near-field of an electromagnetic source, and an “antenna” is reserved for those radiation structures that are intended to be predominantly far-field devices. The subject of the present disclosure includes that of electromagnetic field probes that operate in the near-field of non-contacting rotary joints.
Conventional antennas and near-field probes exhibit a variety of behaviors that preclude or compromise their use in non-contacting rotary joint systems when operating at greater than 1 Gbps data transmission rates. Such rotary joint systems require ultra-wideband (“UWB”) frequency response to pass the necessary frequency components of multi-gigabit digital data, as well as exhibiting high return loss and low distortion impulse response to preserve the time-domain characteristics of the signal. In addition, non-contacting rotary joints exhibit characteristics that complicate the design of antennas and field probes required to capture the energy transmitted across a rotary gap. Typically, non-contacting rotary joints exhibit field strength variations with rotation between the rotor and stator, exhibit directional behavior as the signals travel as waves in transmission lines from the signal source to the transmission line terminations, and may even be discontinuous in the near-field. High-frequency non-contacting rotary joints present a unique set of challenges for the design of near-field probes.
An ideal probe in an ultra-wideband non-contacting rotary joint application should meet seven criteria for successful operation at high data rates. It should:                (1) capture sufficient energy for an acceptable signal-to-noise ratio;        (2) possess bandwidth sufficient to accommodate the major frequency components of the signal;        (3) exhibit high return loss to control internal reflections and preserve signal integrity;        (4) exhibit low distortion impulse response to support good signal integrity;        (5) accommodate nulls in the transmitter pattern while delivering a stable signal;        (6) accommodate the directional responses of the rotary joint while maintaining a stable output signal; and        (7) ameliorate the probe's own directional effects while maintaining the foregoing requirements.        
Conventional prior art antennas and near-field probes generally fail one or more of the foregoing requirements. Most prior art antennas and probes are narrowband standing-wave devices that lack both the frequency response and time-domain response to accommodate the wideband energy of multi-gigabit data streams. Small near-field voltage and current probes may exhibit reasonable frequency and impulse response, but lack a sufficient capture area for an acceptable signal-to-noise ratio. Modern planar patch and bowtie UWB antennas exhibit most of the desirable characteristics for a near-field probe, but, like other prior art antennas and probes, do not inherently address the directional characteristics of non-contacting rotary joints, while simultaneously contending with nulls or discontinuities in the radiation pattern. Further, most antennas and near-probes exhibit directional behaviors of their own at high frequencies. This directional coupler effect further compounds the problems associated with the directionality of non-contacting rotary joints. The combination of effects described above is manifested as variations in signal output from typical near-field probes, can exceed 20 dB, and can present significant challenges for signal recovery.
Addressing all of these requirements simultaneously is the subject of the present invention. The present invention expands the art and addresses the shortcomings of prior rotary joint solutions. The present invention exhibits the following characteristics, and provides:                (1) a high-speed rotary joint, with no electrical contacts in the signal path; and        (2) that ameliorates the directional characteristic of frequency probes and antennas at high frequencies; and        (3) that accommodates a discontinuous field response (nulls) in rotary joints; and        (4) that possesses a good capture area for a high signal-to-noise ratio; and        (5) that has acceptable return loss; and        (6) that exhibits an ultra-wide bandwidth frequency response up to 40 GHz; and        (7) is capable of supporting data transmission rates of greater than 10 gigabits per second.        